A flat object unstacking device comprises four main units:
a mail feed magazine, PA1 a mail holding device, PA1 a double pick separator, PA1 a transfer or take-off device.
The mail is stood on edge on a belt with its back against a plate. The belt and the plate are driven so that the mail is moved towards the holding device. The belt leads to a sudden change in level, referred to hereinafter as the drop, and the mail then reaches a feed magazine between the conveyor belt and the holding device.
The holding device has suction areas across which a perforated belt passes.
On leaving the holding device the mail passes in front of the suction area of the double pick separator on the opposite side of the belt. This unit retains mail items that might otherwise be entrained by friction by the preceding mail item so that only one mail item at a time can leave.
On leaving the double pick separator the mail is taken into a transfer or take-off device which constitutes an interface between the unstacking device and a sorting device.
French patent application No 87 08519 in the name of this Applicant filed 18 Jun. 1987 discloses a holding device which has a sucker for grasping the first flat object from a stack of such objects offered up to it in order to move it to an exit position at which the flat object grasped is released.
The sucker described is a bellows type sucker made from a material such that when it has grasped the first flat object from the stack it is retracted by the effect of the suction and entrains with it the surface of the grasped object. This holding device has the disadvantage of a limited throughput.
French patent application No 91 09431 in the name of this Applicant filed 25 Jul. 1991 discloses a holding member comprising a perforated belt guided and driven by pulleywheels. This holding member comprises a continuously operating suction chamber for prepositioning mail items to be unstacked and a suction head in the vicinity of a double pick separator device and a transfer device. This document also teaches the progressive running up to speed of the unstacked mail item in two stages to reduce the suddenness of unstacking, and this enables open mail items to be processed. A transfer unit and a double pick separator unit comprising retaining means are disposed in the immediate vicinity of the holding unit and the retaining means are actuated when the front edge of a mail item is taken up by the transfer unit.
U.S. Pat. No. 4,357,007 granted 2 Nov. 1982 discloses a holding device comprising two suction devices disposed behind a perforated belt and two sensors adapted to activate one of the suction devices according to the position of the first unstacked mail item.
In processing thin and floppy mail items the problem arises of the antagonistic actions of the holding device and the retaining means of the double pick separator. This can crumple a floppy flat object. It is therefore important for the mail items to be held for as long as possible as they move past the double pick separator.